We present simulations, which predict significantly higher laser to x-ray efficiencies than those previously found in high-intensity (10 20 -10 22 W cm À2 ) laser-solid simulations. The bremsstrahlung emission is shown to last for 10-100 ps, which is difficult to model with conventional particle-in-cell (PIC) codes. The importance of collective effects is also demonstrated, showing the limitations of Monte Carlo modeling in these systems. A new, open-source hybrid-PIC code with bremsstrahlung routines has been developed to model this x-ray production in 3D. Special boundary conditions are used to emulate complex electron refluxing behavior, which has been characterized in 1D and 2D full-PIC simulations. The peak x-ray efficiency was recorded in thick gold targets, with ð7:4 6 1:0Þ% conversion of laser energy into x-rays of energy 1 MeV or higher. The target size is shown to play a role in the conversion efficiency and angular distribution of emitted x-rays, and a simple analytic model is presented for estimating these efficiencies.
Quantum electrodynamics (QED) effects in intense laser plasma interaction were investigated using Particle in Cell (PIC) simulations, specifically the generation of electron-positron pairs. Linearly polarized intense laser pulses were used to irradiate a thin foil (1 μm) with an intensity of 4×10 23 Wcm -2 . A scan of targets with varying Z (Al, Cu and Au) is investigated to determine the effect of target Z/density on electron-positron pair production. The total number of pairs created for Al and Cu targets is 10 14 and 10 13 respectively. In the case of Au, we did not observe any pair production to occur. We have also calculated the variation in electron energy in these targets. The results indicate that target Z plays a very important role with the laser interaction in the pair production process, which will be explained in this paper.
We investigate the production of intense ү-rays following the interaction of ultraintense laser pulse with a hybrid combination of under-dense plasma associated with a thin foil of fully ionized Al or Cu or Au at the rear side. Relativistic electrons are accelerated following the interaction of high intensity laser pulses with an under-dense plasma. These electrons are then stopped by the thin foils attached to the rear side of the under-dense plasma. This results in the production of intense-ray bursts. So, the enhancement of photon generation is due to the under-dense plasma electrons interacting with different over-dense plasma. Using open-source PIC code EPOCH, we study the effect of different electron densities in the under-plasma on photon emission. Photon emission enhancement is observed by increasing the target Z in the hybrid structure. Hybrid structure can enhance photon emission; it can increase the photon energy and yield and improve photon beam divergence. Simulations were also performed to find the optimal under-dense plasma density for ү-ray production.
A novel electron-impact ionization algorithm has been designed for use in particle-in-cell codes. This improved model uses a combination of modified-Bell and relativistic-binary-encounter-Bethe cross sections for greater accuracy and samples the secondary electron kinetic energies from a differential cross section. The algorithm also conserves the number of real-particle ionization events for arbitrary weighted macro-particles, while ionization schemes in existing algorithms are shown to break this condition. Further limitations of existing models have been explored, and the improved model is described within this framework. Benchmarks have been provided to demonstrate the accuracy of this new model.
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